Just off the top of my head
The pill is reversible once you come off it
The pill has been in use, tested, side effects reported for nearly 70 years so is a well understood prescription that is as safe as possible
The pill is used for what it was designed for, puberty blockers for trans children are off-label prescription
There are still controls over prescription of the pill - for younger teens it would be prescribed for a medical necessity (i.e. heavy periods), or, I'd like to think, if purely for contraception would be subject to some scrutiny - i.e. if a 12 year old rocked up to their GP asking for it I'd hope a few questions would be asked. It's not as if it's available in a sweetie jar in a pharmacy for children to help themselves
slightly off topic, but according to wikipedia, which I thought was supposed to be very regulated on contentious topics, puberty blockers are apparently safe and reversible.